The present invention relates to the general field of systems for fixing a fan to a turbojet. It relates more particularly to a system for decoupling a fan from a turbojet in the event of large forces on the fan support.
In flight, a turbojet receives air through a fan. The fan serves to compress a quantity of this air which contributes to the thrust from the turbojet. It comprises a plurality of blades which perform said compression and it is set into rotation by a turbine via a drive shaft. The drive shaft is mounted on bearings which are connected to stationary portions of the turbojet.
Under extreme circumstances, such as losing a blade, a high level of unbalance is generated. Such unbalance gives rise to cyclical loading and vibration, which the bearings supporting the fan drive shaft communicate to the stationary portions of the turbojet, with significant risk of damage.
In order to provide some protection against such danger, it is generally possible to use a decoupling system which allows the fan to be decoupled from the stationary portions of the turbojet in the event of high levels of force. That solution consists in supporting the drive shaft of the fan by a series of links that are suitably placed and calibrated so as to break under a determined load, the fan and its drive shaft then becoming decoupled from the stationary portions of the turbojet. The links are implemented in the form of screws and nuts which are dimensioned so as to break in shear or in bending.
Such a decoupling system nevertheless presents numerous drawbacks. In particular, it implies that the links are broken by direct transmission of the radial force coming from the unbalance of the fan drive shaft, which means there is a high degree of uncertainty as to the exact moment at which the links will break. In order to calculate the decoupling load it is necessary to take account of numerous parameters (in particular the static and dynamic forces applied to the turbojet), and these parameters vary, both during operation and over the lifetime of the turbojet. These varying parameters are not taken into consideration by conventional decoupling systems when calibrating the decoupling load. In addition, in certain situations in flight, it is sometimes preferable not to decouple the fan, but instead to reduce the thrust from the turbojet in order to remain below the decoupling load.